Aluminum beverage containers are generally made in two pieces, one piece forming the container sidewalls and bottom (referred to herein as a "container body") and a second piece forming a container top. Container bodies are formed by methods well known in the art. Generally, the container body is fabricated by forming a cup from a circular blank aluminum sheet (i.e., body stock) and then extending and thinning the sidewalls by passing the cup through a series of dies having progressively smaller bore sizes. This process is referred to as "drawing and ironing" the container body. The ends of the container are formed from end stock and attached to the container body. The tab on the upper container end that is used to provide an opening to dispense the contents of the container is formed from tab stock.
Aluminum alloy sheet is most commonly produced by an ingot casting process. In the process, the aluminum alloy material is initially cast into an ingot, for example, having a thickness ranging from about 20 to about 30 inches. The ingot is then homogenized by heating to an elevated temperature, which is typically 1075.degree. F. to 1150.degree. F., for an extended period of time, such as from about 6 to about 24 hours. "Homogenization" refers to a process whereby ingots are raised to temperatures near the solidus temperature and held at that temperature for varying lengths of time. The process reduces microsegregation by promoting diffusion of solute atoms within the grains of alumina and improves workability. Homogenization does not alter the crystal structure of the ingot. The homogenized ingot is then hot rolled in a series of passes to reduce the thickness of the ingot. The hot rolled sheet is then cold rolled to the desired final gauge.
Although ingot casting is a common technique for producing aluminum alloy sheet, a highly advantageous method for producing aluminum alloy sheet is by continuously casting molten metal. In a continuous casting process, molten metal is continuously cast directly into a relatively long, thin slab and the cast slab is then hot rolled and cold rolled to produce a finished product.
Some alloys are not readily cast using a continuous casting process into an aluminum sheet having mechanical properties suitable for forming operations, especially for making drawn and ironed container bodies. By way of example, some alloys have low yield and tensile strengths, a low degree of formability and/or a high earing which lead to a number of problems.
It would be desirable to have a continuous aluminum casting process in which the aluminum alloy sheet can be readily fabricated into desired objects. It would be advantageous to have a continuous casting process in which the aluminum alloy sheet has a high degree of formability, low earing and high strength.